Dc pre-charge circuit

ABSTRACT

Systems and methods are provided for pre-charging the DC bus on a motor drive. Pre-charging techniques involve pre-charge circuitry including a manual switch, an automatic switch, and pre-charge control circuitry to switch the automatic switch between pre-charge and pre-charge bypass modes in response to an initialized pre-charge operation, input voltage sags, and so forth. In some embodiments, the pre-charge operation may be initialized by switching the manual switch closed. In some embodiments, the pre-charge operation may also be initialized by a detected voltage sag on the DC bus. The pre-charge circuitry may also be configured to disconnect to isolate a motor drive from the common DC bus under certain fault conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/559,650, filed Nov. 14, 2011.

BACKGROUND

The invention relates generally to the field of electrical drives. Moreparticularly, the invention relates to techniques for pre-charging adirect current (DC) bus in a power drive.

In the field of power electronic devices, a wide range of circuitry isknown and currently available for converting, producing, and applyingpower to loads. Depending upon the application, motor drive systems mayinclude circuitry that converts incoming power from one form to anotheras needed by the load. In a typical drive system, for example, arectifier converts alternating current (AC) power (such as from autility grid or generator) to direct current (DC) power. Invertercircuitry can then convert the DC signal into an AC signal of aparticular frequency desired for driving a motor at a particular speed.The inverter circuitry typically includes several high power switches,such as insulated-gate bipolar transistors (IGBTs) that are controlledby drive circuitry. Motor drive systems also often include powerconditioning circuitry, including capacitors and/or inductors, whichremove undesirable ripple currents from its DC bus.

Sometimes during operation of a motor drive system, and particularlyduring start-up, high levels of in-rush current may be received by amotor drive in the motor drive system, which may cause various adverseaffects to the motor drive. To avoid these high levels of in-rushcurrent during start-up, a typical motor drive system may includepre-charge circuitry that applies a smaller initial current to the DCbus of the motor drive system prior to actually starting the motordrive. The pre-charge circuitry charges a number of capacitors coupledto the inverter before applying a full source voltage to the inverter.Such techniques may be referred to as pre-charging the DC bus.

Typical pre-charge techniques may include increasing a firing angle ofsemiconductor devices (e.g., thyristor) in a rectifier until capacitorson a DC bus are charged to some level or connecting a resistor with acontactor in parallel such that the resistor is bypassed via thecontactor after the DC capacitors are charged. Other pre-chargecircuitry may involve a three-way switch, which may connect the DC bus,pre-charge the DC bus, or disconnect the DC bus. However, suchconfigurations may result in significant power loss through the diodeand may not be used to isolate the drive from the common DC bus.Furthermore, each of the above described pre-charge circuitconfigurations and techniques involve using one or more circuit breakersto isolate or disconnect a drive from the common DC bus, which increasesthe overall size of each drive.

BRIEF DESCRIPTION

The present invention relates generally to techniques for pre-charging aDC bus on a motor drive system. Specifically, pre-charge circuitryincludes a manual switch, an automatic switch, and pre-charge controlcircuitry to switch the automatic switch between a pre-charge mode and apre-charge bypass mode. In some embodiments, the pre-charge mode may beinitialized by manually closing the manual switch. Once the pre-chargeoperation is complete, the automatic switch may be automatically closedby the pre-charge control circuitry, and the inverter circuitry of themotor drive may begin to operate. In certain embodiments, the pre-chargecontrol circuitry may detect various fault conditions, such as voltagedrops in the DC bus and/or faults within the motor drive systems. Whensuch conditions are recognized, the pre-charge control circuitry mayopen the automatic switch such that the motor drive system is in itspre-charge mode, and the DC bus may be recharged during voltage drops.Further, by opening the automatic switch, the motor drive system mayisolate its drive from the common DC bus to protect its drive from thevarious fault conditions.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical representation of a power distributionnetwork, in accordance with an embodiment;

FIG. 2 is a diagrammatical representation of pre-charge circuitry in amotor drive of FIG. 1, in accordance with an embodiment;

FIGS. 3A-3B illustrate a flow chart representing a process forpre-charging a capacitor on the DC bus in the motor drive of FIG. 2, inaccordance with an embodiment;

FIGS. 4A-4B illustrate a flow chart representing a process for resettingthe pre-charge circuitry of FIG. 2 when a DC voltage drop is detected,in accordance with an embodiment;

FIGS. 5A-5B illustrate a flow chart representing a process for isolatinga motor drive of FIG. 1, in accordance with an embodiment;

FIG. 6 is a diagrammatical representation of pre-charge circuitry in amotor drive of FIG. 1, in accordance with an embodiment;

FIG. 7 is a diagrammatical representation of an undervoltage delayelement in pre-charge circuitry of FIG. 6, in accordance with anembodiment; and

FIG. 8 is a diagrammatical representation of a power distributionnetwork having a switch shared by multiple motor drives, in accordancewith an embodiment.

DETAILED DESCRIPTION

Systems and methods of the present invention are related to pre-charginga DC bus on a motor drive in a motor drive system. During operation of amotor drive system, and particularly during start-up, motor drivecircuitry (e.g., inverter, capacitors) may draw high levels of in-rushcurrent while charging power-conditioning capacitors associated with themotor drive. Typically, motor drive configurations include pre-chargecircuitry, which applies a smaller initial current to the DC bus of themotor drive prior to starting the motor drive. The smaller currentprovided to the DC bus may charge DC capacitors (i.e.,power-conditioning capacitors) that may be coupled to the DC bus.Pre-charging the capacitors with the smaller initial current may protectthe capacitors from possible damage that may be caused by the highlevels of in-rush current during start-up. Some existing pre-chargetechniques may not be suitable for addressing line sags (e.g., drops inthe DC bus voltage) which may occur after the motor drive has begun tooperate. Moreover, some existing techniques may not isolate the motordrive from various faults, which may damage the motor drive. Embodimentsof the present disclosure include pre-charge circuitry that may besuitable for addressing line sags during drive operation and forisolating a drive from a common DC bus under certain fault conditions.Furthermore, in some embodiments, the pre-charge circuitry configurationmay utilize automatic switches, such as molded case switches, to providedrive isolation without the use of a circuit breaker. Such embodimentsmay be sufficiently compact to fit in a single cabinet in a motor drivesystem.

FIG. 1 is a diagrammatical representation of a power distributionnetwork 10 in accordance with an embodiment of the techniques describedherein. The power distribution network 10 may include one or more motordrives 12 coupled to a common bus 14 through a DC bus 32 having a highside (+v) 34 and low side (−v) 36. As shown in FIG. 1, the DC bus 14 mayprovide power to several motor drives 12. It should be noted, however,that in some embodiments the DC bus 14 may be dedicated to a singlemotor drive 12. Furthermore, the DC bus 14 may be powered by any DCsource known in the art, such as a battery, a solar panel, or arectified AC source. In some embodiments the DC bus 14 may be powered byan 18-pulse, active front-end rectifier.

The motor drive 12 may include an inverter 22 that generates a threephase output waveform at a desired frequency for driving a motor 30connected to the output terminals 24, 26 and 28. The motor drive 12 mayalso include a capacitor 38 connected between the high side 34 of the DCbus 32 with the low side 36 of the DC bus 32. In some embodiments, thecapacitor 38 may be configured to smooth the DC voltage waveform myremoving AC ripples from the common DC bus 14 such that the internal DCbus may carry a waveform closely approximating a true DC voltage.

In some embodiments, the motor drive 12 may include a pre-charge circuit40 that reduces the in-rush current that may otherwise occur when poweris first applied to the motor drive 12. A high in-rush current can befacilitated, in part, by the capacitor 38, which may briefly behave likea short-circuit after voltage is applied to the local DC bus 32 andbefore the capacitor 38 has stored sufficient charge. Generally, thepre-charge circuit 40 may reduce in-rush current by controlling aninitial charging current to the capacitor 38 during an initial chargingstage in which the capacitor 38 charges to the approximate bus voltage.

In some embodiments, each motor drive 12 may include a fuse 20 on eachof the high side 34 and low side 36 of the DC bus 32. The fuse 20 may besuitable for providing load breaking for elements in the pre-chargecircuit 40, as will be discussed in greater detail. In some embodiments,the fuse may be a resistor or any suitable sacrificial device thatprovides current protection (e.g., during a short circuit) to pre-chargecircuit elements.

FIG. 2 is a diagrammatical representation of a portion of a motor drive12 illustrated in FIG. 1 that employs pre-charge circuitry 40 inaccordance with an embodiment of the present techniques. The motor drivemay include an inverter 22 having an arrangement of solid state switches42, such as power metal-oxide semiconductor field-effect transistors(MOSFETs) or insulated gate bipolar junction transistors (IGBTs), forexample. In some embodiments, a pair of switches 42 may be coupled inseries, collector to emitter, between the high side 34 and the low side36 of the internal DC bus 32. In the illustrated embodiment of FIG. 2,three of these pairs of switches are coupled in parallel to the internalDC bus 32 for a total of six switches 42. Each switch 42 is arranged inparallel and in opposite direction to a diode 44, where the collector ofthe switch 42 is coupled to the anode and the emitter of the switch 42is coupled to the cathode. Each of the output terminals 24, 26, and 28is coupled to one of the switch outputs between one of the pairs ofswitches 42. Furthermore, driver circuitry 46 may be coupled to theinverter 22 to generate a three-phase output waveform. In someembodiments, driver circuitry 46 may be connected to each of theswitches 42 to cause the switches 42 to switch rapidly in a particularsequence so as to generate an approximately sinusoidal output waveform.

The inverter 22 may be connected to the internal DC bus 32 in parallelwith a capacitor 38 and with the pre-charge circuitry 40. The pre-chargecircuitry 40 may include manual switches 50. The manual switches 50 mayinclude a high side manual switch 54 connected in series to pre-chargeresistor 58 and parallel to the high side 34 of the DC bus 32. Themanual switches 50 may also include a low side manual switch 52connected in series to pre-charge resistor 56 and parallel to the lowside 36 of the DC bus 32. The pre-charge circuitry 40 also includesautomatic switches 60 including a high side automatic switch 62 in thehigh side 34 of the DC bus 32 and a low side automatic switch 64 in thelow side 36 of the DC bus 32. The automatic switches 60 may becontrolled by pre-charge control circuitry 66 in the pre-chargecircuitry 40. The pre-charge control circuitry 66 may include acommunication component, a processor, a memory, a storage, input/output(I/O) ports, and the like. The communication component may be a wirelessor wired communication component that may facilitate communicationbetween the pre-charge control circuitry 66, the manual switches 50, theautomatic switches 60, and the like. The processor may be any type ofcomputer processor or microprocessor capable of executingcomputer-executable code. The memory and the storage may be any suitablearticles of manufacture that can serve as media to storeprocessor-executable code. These articles of manufacture may representcomputer-readable media (i.e., any suitable form of memory or storage)that may store the processor-executable code used by the processor toperform the presently disclosed techniques.

In some embodiments, the automatic switches 60 may be a motorized moldedcase switch, and the pre-charge control circuitry 66 may control theopening or closing of the automatic switches 60 by applying power to acoil or motor of the automatic switches 60. The pre-charge controlcircuitry 66 may be connected to each side of the pre-charge resistors56 and 58 to sense the voltage drop across the pre-charge resistors 56and 58. Based on the sensed voltage drop, the pre-charge controlcircuitry 66 may control the opening or closing of the automaticswitches 60. In some embodiments, the pre-charge control circuitry 66may also include a transformer 68 configured to provide power to variouspre-charge operations. For instance, the transformer 68 may power one ormore coils or motors for closing or opening the automatic switches 60.

Generally, while the capacitor 38 is charging during a pre-chargeoperation, the manual switches 50 are closed while the automaticswitches 60 are open, and the pre-charge control circuitry deliverspower through the closed manual switches 50. Because the automaticswitches 60 are open, the pre-charge current may flow through the closedmanual switches 50 and the pre-charge resistors 56 and 58, and thecurrent draw on the DC bus 32 may be controlled to an acceptable valueknown in the art while the capacitor 38 charges. After a suitable timeperiod has elapsed, or after a threshold voltage on the DC bus 32 hasbeen met (as measured across the pre-charge resistors 56 and 58), thepre-charge control circuitry 66 may close the automatic switches 60 tobypass the pre-charge resistors 56 and 58, thereby automaticallydisconnecting the pre-charge resistors 56 and 58 from the motor drive12. Because the capacitor 38 will have been charged to a voltage closeto the DC bus voltage, excessive in-rush currents may be substantiallyavoided.

In some embodiments, a fuse 20 may be configured on each of the highside 34 and low side 36 of the DC bus 32 between the manual switches 50and the automatic switches 60. For example, a first fuse 20 may bebetween the high side automatic switch 62 and the high side manualswitch 54 and a second fuse 20 may be between the low side automaticswitch 64 and the low side manual switch 52. The fuse 20 may be anysuitable element (e.g., a resistor, a sacrificial wire) suitable forproviding load breaking for the automatic switches 60. In someinstances, such as during a short circuit, the fuse 20 may provide adisconnect means for the automatic switches 62 and 64 to protect theautomatic switches 62 and 64. Furthermore, in some embodiments, fuses 20may also be configured with the manual switches 50 to protect the manualswitches from influxes of current.

More detailed explanations of various embodiments for operating thepre-charge circuitry 40 in a motor drive 12 are discussed in the flowcharts of FIGS. 3-5. FIGS. 3A-3B illustrate a flow chart representinginitializing a pre-charge operation. FIGS. 4A-4B and 5A-5B illustrateflow charts representing processes for resetting the pre-chargecircuitry 40 in various fault conditions. More specifically, FIGS. 4A-4Billustrate a flow chart for resetting the pre-charge circuitry 40 if adrop in the DC bus voltage is detected. FIGS. 5A-5B illustrate a flowchart for isolating the drive 12 from the common DC bus 14 if the manualswitch is opened during drive operation or if the pre-charge circuitrytransformer turns off. As the flow charts of FIGS. 3A-5B refer tocomponents discussed with respect to FIGS. 1 and 2, the flow charts ofFIGS. 3A-5B may each be discussed concurrently with FIGS. 1 and 2.Furthermore, as used herein, the high side manual switch 54 and low sidemanual switch 52 may be referred to generally as the manual switch 50.Similarly, the high side automatic switch 62 and the low side automaticswitch 64 may be referred to generally as the automatic switch 60,though in some embodiments, the pre-charge control circuitry 66 maycontrol each of the high side and low side automatic switches 62 and 64independently.

Beginning first with FIGS. 3A-3B, a DC pre-charge operation may begin(block 72) when the motor drive 12 is off and any previously detectedfaults have been reset. In this condition, the automatic switch 60 maybe open. The pre-charge control circuitry 66 may determine (block 74)whether the manual switch 50 is closed. In some embodiments, the manualswitch 50 may be closed or opened by an operator of the motor drive 12or the power network 10, and the manual switch 50 may be manually closedby the operator before activating the drive inverter 22 of a motor drive12. If the manual switch 50 is closed, a pre-charge operation may beinitialized, and the pre-charge control circuitry 66 may start (block76) a pre-charge timer, which may be set to a period of time in whichthe DC bus is charged to an appropriate voltage. To pre-charge theinternal DC bus 32 of the motor drive 12, the pre-charge controlcircuitry 66 may draw power from the common DC bus 14 until a suitablethreshold voltage is drawn to the internal DC bus 32. As the capacitor38 is connected across the internal DC bus 32, reaching a suitablethreshold voltage on the internal DC bus 32 may indicate that thecapacitor 38 has been appropriately charged. For example, the thresholdmay be approximately 700V in some embodiments, though the threshold maybe different depending on different types of motor drives 12 or drivenetworks 10. In some embodiments, during pre-charging, current flowsthrough the pre-charge resistors 56 and 58. As such, the current draw onthe DC bus 32 may be controlled while the capacitor 38 charges.

The pre-charge control circuitry 66 may measure the DC bus voltage todetermine (block 78) whether the DC bus voltage is above the threshold,indicating that pre-charging is complete. The control circuitry 66 maycontinue to detect the DC bus voltage until the pre-charge timer istimed out. If the control circuitry 66 determines (block 80) that thepre-charge timer has timed out while the threshold voltage has not beenmet, the control circuitry 66 may set (block 82) the pre-chargeoperation to a fault condition, as an appropriate DC bus voltage has notbeen reached within the pre-charge timer period. In some embodiments, apre-charge fault may indicate to an operator that one or more componentsof the pre-charge operation must be repaired, replaced, and/or reset.The pre-charge fault may be reset (block 84) to restart the pre-chargeoperation 70.

In some embodiments, if the pre-charge control circuitry 66 determines(block 78) that the DC bus voltage is greater than the threshold, thecontrol circuitry 66 may proceed to close the automatic switch to beginoperating the motor drive in a pre-charge bypass mode. The controlcircuitry 66 may determine (block 86) whether the transformer 68 coupledto the control circuitry 66 is on. In some embodiments, the controlcircuitry 66 may use the transformer 68 to supply power for controllingthe switching of the automatic switch 60. If the transformer 68 is on,the control circuitry 66 may power (block 88) an under voltage (UV) coilat the automatic switch 60. The UV coil may be coupled to motors of theautomatic switch 60, and by powering the UV coil, the control circuitry66 may substantially control the operations of the automatic switch 60.The control circuitry 66 may also charge (block 90) a motor operatorcoil of the automatic switch 60 to a charge sufficient for closing(block 92) the automatic switch 60. In some embodiments, charging themotor operator coil may involve applying power to the motor operatorcoil for a period of time and removing the power once charging iscomplete.

The control circuitry 66 may verify (block 94) that the automatic switch60 has been closed. If the automatic switch 60 has not been properlyclosed, the control circuitry may remove (block 96) power from the UVcoil and set (block 98) the automatic switch at fault. Such a fault mayindicate to an operator that the automatic switch needs repair orattention. If the control circuitry 66 determines (block 94) that theautomatic switch 60 has properly closed, the control circuitry 66 mayindicate a successful pre-charge operation, and the motor drive 12 maybe activated in a pre-charge bypass mode (block 100), where theautomatic switch 60 is closed, and the inverter circuitry 22 isoperating in the motor drive 12.

FIGS. 4A-4B illustrate a flow chart representing a process for resettingthe pre-charge circuitry 40 if a drop in the DC bus voltage is detectedwhile the motor drive 12 is operating. As used herein, a motor drive 12having a drop in DC bus voltage during operation may be referred to as afaulted drive. The pre-charge reset process 110 may begin (block 112)when the motor drive 12 is on and the pre-charge control circuitry 66determines (block 114) that the DC bus voltage has dropped below aminimum threshold. In some embodiments, if the DC bus voltage dropsbelow the minimum threshold, various components may be susceptible todamage, particularly for high power DC input drives. The minimumthreshold may depend on various factors, such as the components used inthe motor drive 12, the current limits of the DC bus 32, and/or otheroperating conditions of the motor drive 12. When a voltage drop isdetected (block 114), either the pre-charge control circuitry 66 or thedriver circuitry 46 or any other suitable controller of the motor drive12 may disable (block 116) the inverter 22 of the motor drive 12. In oneembodiment, the driver circuitry 46 may disable the inverter 22 bystopping the switching of the switches 42.

After disabling the inverter 22, the pre-charge control circuitry 66 mayapply (block 118) power to a shunt trip coil to open the automaticswitch 60. The pre-charge control circuitry 66 may verify (block 120)whether the automatic switch 60 is open. If the automatic switch 60 isopen, the control circuitry 66 may determine that the drop in DC voltageis due to a fault in the UV coil, and the control circuitry 66 may set(block 122) a bus UV fault. In some embodiments, the pre-chargecircuitry 40 may be suitable for recharging the DC bus after a voltagedrop. As the manual switch 50 is closed, the faulted drive may draw DCvoltage from the common bus 14 until the voltage of the common DC bus 32is again above the threshold discussed in FIGS. 3A-3B (e.g., 700V). Oncethe control circuitry 66 determines (block 124) that the DC bus voltageis above the threshold, the control circuitry 66 may reset (block 126)the bus UV fault, and the motor drive 12 may be turned off (block 128).In one embodiment, the control circuitry 66 may then turn the motordrive 12 by following the pre-charge operation 70 described above withreference to FIGS. 3A-3B.

If the pre-charge control circuitry determines (block 120) that theautomatic switch is still closed, the control circuitry 66 may remove(block 130) power from the UV coil and set (block 132) a fault for theautomatic switch 60. In some embodiments, the control circuitry 66 mayprovide an indication for an operator to open the manual switch 50. Oncethe control circuitry 66 determines (block 134) that the manual switch50 is open, the control circuitry 66 may reset (block 136) the automaticswitch fault and any other faults. The process 110 may end (block 128)with the motor drive 12 turned off and the faults reset. The motor drive12 may then be prepared for pre-charge mode (e.g., performing theprocess 70 in FIGS. 3A-3B).

FIGS. 5A-5B illustrate a flow chart representing a process for isolatingthe drive 12 from the common DC bus 14 if the manual switch 50 isswitched open or if the transformer 68 is turned off during operation ofthe drive 12. Typically, the manual switch 50 may remain closed throughpre-charge and after the drive 12 is in operation. A manual switch 50which opens during drive operation may prevent the drive 12 fromautomatically pre-charging if voltage drops are detected. Further, ifthe transformer 68 loses power during drive operation, the pre-chargecontrol circuitry 66 may not control the automatic switch 60 in responseto voltage drops. Therefore, in some embodiments, an open manual switch50 or a faulty transformer 68 may be fault conditions that prevent thedrive 12 from properly pre-charging. As such, a motor drive 12 havingconditions that may prevent the drive 12 from properly pre-charging maybe isolated from the common DC bus 14.

The drive isolation process 140 may begin (block 142) when the drive 12is on and the pre-charge control circuitry 66 determines (block 144)that the manual switch 50 is open. Furthermore, the control circuitry 66may also determine (block 146) whether the transformer 68 is poweredoff. If the control circuitry 66 determines (blocks 144 and 146) thateither the manual switch is open or the transformer 68 power is off, thecontrol circuitry 66 may disable (block 148) the drive 12 to protect thedrive 12 from possible voltage fluctuations on the DC bus.

Once the drive 12 is disabled, the control circuitry 66 may then apply(block 150) power to the shunt trip coil to open the automatic switch60. The control circuitry 66 may determine (block 152) whether theautomatic switch 60 is open. If the automatic switch 60 is open, thecontrol circuitry 66 may set (block 156) a shunt trip fault. The shunttrip fault may indicate to an operator that the shunt trip has beenpowered to open the switch and break the DC bus at the automatic switch60. If the control circuitry 66 determines (block 152) that theautomatic switch 60 is not open, the control circuitry 66 may remove(block 158) power from the UV coil and set (block 160) an automaticswitch fault, indicating to an operator that the automatic switch is infault because it is still closed.

Once either of the shunt trip or automatic switch faults are set (blocks156 and 160), the control circuitry 66 may determine (block 162) whetherthe manual switch 50 is open. In some embodiments, the control circuitry66 may provide an indicator that the faulted drive has one or more faultconditions, and in some embodiments, the control circuitry 66 mayprovide indication for an operator to open the manual switch 50. Oncethe manual switch 50 is open, the control circuitry 66 may reset (block164) the faults of the drive 12, and the process 140 may end (block 166)with the drive off, the faults reset, and the drive 12 isolated from theDC bus 14.

In some embodiments, the control circuitry 66 may output the faults(e.g., automatic switch fault, pre-charge fault, bus UV fault, shunttrip fault, etc.) to an operator (e.g., via a display or saved in memoryto be retrieved by the operator), such that the operator may address oneor more fault conditions. For instance, based on the fault conditionsoutput to an operator, the operator may replace the automatic switch 60,replace the transformer 68, or repair the pre-charge circuitry 40, etc.

In some embodiments as discussed in FIGS. 4A-4B, using a configurationwith the automatic switch 60 and the manual switch 50, a motor drive 12may automatically switch from a pre-charge bypass mode during normaldrive operations to a pre-charge mode when DC bus voltage drops aredetected. Therefore, because the switch to pre-charge mode may besubstantially automatic, an operator need not constantly monitormultiple motor drives 12 for voltage drops or other faults. Byautomatically switching to pre-charge mode during DC voltage drops, themotor drive 12 may recover safely without causing substantial harm tomotor drive components (e.g., the capacitor 38, the switches 42, etc).If the fault condition cannot be cured by operating the motor drive 12in pre-charge mode, a motor drive 12 in fault may be isolated from acommon bus 14, such that other motor drives 12 not in fault may not beharmed and/or may continue to operate in the power system 10. Moreover,due to the relatively small size of the automatic switch 60, the size ofthe motor drive 12 may be reduced, as the drive 12 may not need to use arelatively larger circuit breaker for drive isolation.

The manual switches 50 may also be automatically protected by thecontrol circuitry 66 in some embodiments. As discussed in the processesfor resetting various fault conditions in FIGS. 4 and 5, determining(blocks 134 and 162, respectively) whether the manual switch 50 is openmay involve providing indication for an operator to open the manualswitch 50 if it is determined to not be open. However, in someembodiments, as illustrated in the pre-charge circuitry 40A of FIG. 6,the pre-charge circuitry 40A may include contactors 170 (contactor 172and contactor 174) connected in series between the manual switches 50and the pre-charge resistors 56 and 58. In some embodiments, afterpre-charge is complete, the automatic switches 60 may be closed by thecontrol circuitry 66. The control circuitry 66 may also open thecontactors 172 and 174. If a fault (e.g., a short circuit) occurs whilethe drive 12 is in operation, the control circuitry 66 may open theautomatic switches 60. If the manual switches 50 are still closed whenthe automatic switches 60 are open during a short circuit, thepre-charge resistors 56 and 58 may be damaged by an influx of current.In some embodiments, the contactors 172 and 174 may be open by thepre-charge control circuitry 66 to open the current path to thepre-charge resistors 56 and 58 and protect the pre-charge resistors 56and 58 from the current influx. The control circuitry 66 may thendetermine a more suitable time (e.g., safer currents on the DC bus forthe pre-charge resistors 56 and 58) to close the contactors 170 andstarting a pre-charge operation.

Furthermore, in some embodiments as illustrated in FIG. 7, thepre-charge circuit 40 may include an undervoltage (UV) delay element 176connected to an undervoltage trip coil of the automatic switches 60.During fault conditions such as a dip or drop in DC bus voltage (e.g.,as discussed with respect to FIGS. 4A-4B), the pre-charge controlcircuitry 66 may disable the drive 12 and open the automatic switches 60as soon as a dip in voltage is detected. For example, if the motor drive12 operates with 240V, a dip beneath the 240V threshold may cause thecontrol circuitry 66 to open the automatic switches 60. However, in someinstances, the automatic switches 60 may not be prepared to open. Forexample, if the inverter 22 has not been fully disabled, opening theautomatic switches 60 may break the current flow while the motor drive12 is supporting a full load, which may cause damage to the automaticswitches 60. The UV delay element 176 may receive an AC voltage of240V_(AC) and output a DC voltage of 240 V_(AC). The 240V_(AC) may beheld for a length of time (e.g., 3 seconds) in such voltage dipconditions to the UV trip coil. As such, the control circuitry 66 mayhave sufficient time to fully disable the inverter 22 before opening theautomatic switches 60, decreasing the possibility of damage to theautomatic switches 60.

In some embodiments, the pre-charge circuit 40 may also include acontactor 178 connected in series with the automatic switch 60. Thecontactor 178 may be controlled via a control signal 180 from thecontrol circuitry 66. The control circuitry 66 may power a coil to openthe contactor 178 and trip the automatic switch 60. In some embodiments,the control circuitry 66 can control the contactor 178 to trip theautomatic switch 60 without a UV delay element 176. Tripping theautomatic switch 60 may provide sufficient time for the inverter 22 tobe disabled. Once the inverter 22 is disabled, the control circuitry 66may open the automatic switch 60.

Furthermore, in some embodiments, the manual switches 50 and automaticswitches 60 are individually coupled to each motor drive 12 in thenetwork 10, as illustrated in FIG. 2. By individually opening the manualand automatic switches 50 and 60 on one drive in fault condition, thefaulted drive may be isolated from the remaining drives in the network10, such that the network 10 may still function, and appropriateattention or repairs may be given to the faulted drive. In otherembodiments, the manual switches 50 and/or the automatic switches 60 maybe connected to more than one motor drive 12 for further space savings.For instance in one embodiment, as illustrated in FIG. 8, one set ofmanual switches 50A may be connected to all the motor drives 12 in thepower network 10A, such that an operator may begin or reset a pre-chargeoperation for all connected motor drives 12 by switching one manualswitch 50A.

While only certain features have been illustrated and described herein,many modifications and changes will occur to those skilled in the art.It is, therefore, to be understood that the appended claims are intendedto cover all such modifications and changes.

1. A power system, comprising: at least one inverter module, wherein theat least one inverter module comprises: an inverter circuit configuredto connect to a direct current (DC) bus; a capacitive circuit configuredto couple across the DC bus; and a pre-charge circuit configured tocharge the capacitive circuit, wherein the pre-charge circuit comprises:at least one pre-charge resistor; at least one manual switch configuredto connect the at least one pre-charge resistor to the DC bus whenclosed and disconnect the at least one pre-charge resistor from the DCbus when open; at least one automatic switch on the DC bus; andpre-charge control circuitry configured to detect a state of the atleast one manual switch and configured to open and close the at leastone automatic switch.
 2. The power system of claim 1, wherein thepre-charge control circuitry is configured to apply a pre-charge currentthrough the at least one pre-charge resistor when a voltage of the DCbus is below a threshold.
 3. The power system of claim 1, wherein thepre-charge control circuitry is configured to open the at least oneautomatic switch when a voltage of the DC bus is below a threshold. 4.The power system of claim 3, wherein the pre-charge control circuitry isconfigured to provide an indication to open the at least one manualswitch when the voltage is below the threshold.
 5. The power system ofclaim 4, wherein the at least one opened automatic switch and the atleast one opened manual switch isolates the at least one inverter modulefrom the power system.
 6. The power system of claim 1, wherein thepre-charge circuit comprises at least one contactor configured toconnect in series between the at least one manual switch and the atleast one pre-charge resistor.
 7. The power system of claim 6, whereinthe pre-charge control circuitry is configured to open the at least onecontactor when a voltage of the DC bus is below a threshold.
 8. Thepower system of claim 1, wherein the pre-charge control circuitry isconfigured to control a flow of current through the at least onepre-charge resistor for a period of time to sufficiently charge thecapacitive circuit to a threshold.
 9. The power system of claim 1,wherein the at least one automatic switch comprises at least onemotorized molded case switch.
 10. The power system of claim 1, whereinthe pre-charge circuit comprises at least one under-voltage delayelement connected to the at least one automatic switch, wherein the atleast one under-voltage delay element is configured to hold a thresholdoperating voltage on the DC bus for a period of time after a voltage inthe DC bus is below the threshold operating voltage.
 11. The powersystem of claim 1, wherein the pre-charge circuit comprises at least onecontactor configured to connect to the at least one automatic switch,wherein the at least one contactor is configured to open based on acontrol signal from the pre-charge control circuitry when a voltage inthe DC bus is below a threshold.
 12. The power system of claim 1,comprising at least one fuse connected on the DC bus, wherein the atleast one fuse is configured to interrupt current flow over the DC busto the at least one automatic switch during a short circuit condition ofthe at least one inverter module.
 13. A method, comprising: receiving afirst signal indicating that a manual switch is closed, wherein theclosed manual switch is configured to couple a DC source in series witha resistor and an internal DC bus, wherein the DC source is coupled to aplurality of inverter modules, each inverter module comprises aninverter circuit, the internal DC bus coupled to the inverter circuit,and a capacitive circuit coupled across the internal DC bus;automatically closing an automatic switch coupled between the DC sourceand the internal DC bus when the capacitive circuit has a voltage abovea threshold.
 14. The method of claim 13, comprising automaticallyopening the automatic switch when a fault condition is present on atleast one of the plurality of inverter modules.
 15. The method of claim13, wherein the first signal indicates that a plurality of switches isclosed, wherein each of the plurality of switches is coupled to arespective internal DC bus in each of the plurality of inverter modules.16. The method of claim 13, wherein the first signal indicates that onemanual switch coupled to each respective inverter module in theplurality of inverter modules is closed.
 17. The method of claim 13,comprising operating the inverter circuit after the automatic switch isclosed.
 18. The method of claim 13, comprising disabling the invertercircuit when a fault condition is present on at least one of theplurality of inverter modules.
 19. The method of claim 18, wherein thefault condition comprises one or more of a significant voltage drop inthe internal DC bus, an opening of the manual switch while the invertercircuit is in operation, or any combination thereof.
 20. The method ofclaim 13, comprising automatically opening a contactor coupled in serieswith the resistor when a fault condition is present on at least one ofthe plurality of inverter modules.
 21. The method of claim 13,comprising automatically opening a contactor coupled in series with thefirst switch when a fault condition is present on at least one of theplurality of inverter modules.
 22. The method of claim 13, comprisingreceiving a second signal that the manual switch has been opened after afault condition present on at least one of the plurality of invertermodules has been removed to reset faults of a respective inverter modulein which the fault condition is detected.
 23. An inverter modulecomprising: a power converter coupled to a first direct current (DC) busand configured to convert DC power from the first DC bus to AC outputpower; a first switch coupled between the first DC bus and a common DCbus; a capacitive circuit coupled across the first DC bus; a secondswitch configured to couple a resistor in series with the common DC busand the first DC bus when closed; and control circuitry configured toautomatically close the first switch when a voltage of the capacitivecircuit is above a first threshold.
 24. The inverter module of claim 23,wherein the control circuitry is configured to automatically open thefirst switch when a voltage of the first DC bus is below a secondthreshold.
 25. The inverter module of claim 24, wherein the controlcircuitry is configured to automatically close the first switch when thefirst DC bus is above the second threshold.
 26. The inverter module ofclaim 23, comprising power converter drive circuitry configured tosubstantially control the power converter, wherein the power converterdrive circuitry is configured to disable the power converter upon thedetection of a fault condition.